1. Field of the Invention
This invention relates to digital video tape recorders, and more particularly to a video tape recording and/or reproducing apparatus, hereinafter a VTR, that may reproduce a video signal from a magnetic tape using a magnetic head that is different in width from a magnetic head with which the tape was recorded.
2. Description of the Prior Art
A D1 format component-type digital VTR and a D2 format composite-type digital VTR have been developed for use by broadcasting stations in digitizing color video signals and recording the digitized signals on a recording medium, such as a magnetic tape. In the D1 format digital VTR, a luminance signal and first and second color different signals are A/D converted with sampling frequencies of 13.5 MHz and 6.75 MHz, respectively. Thereafter, the signals are suitably processed and then recorded on a magnetic tape. Since the ratio of sampling frequencies of the signal components is 4:2:2, this system is usually referred to as the 4:2:2 system.
On the other hand, in the D2 format video digital VTR, a composite video signal is sampled at a sampling rate that is four times higher than the frequency fsc of a color sub-carrier signal and then is A/D converted. Thereafter, the resultant signal is suitably processed and then recorded on a magnetic tape.
Since these known D1 and D2 format digital VTRs are designed for professional use, for example in broadcasting stations, attainment of high picture quality is given top priority in the design and construction of such VTRs, and the weight and size of the apparatus is not overly important.
In these known digital VTRs, the digital color video signal, which results from each sample being A/D converted into, for example, 8 bits, is recorded without being substantially compressed. As an example, in the known D1 format digital VTR which A/D converts each sample into 8 bits with the frequencies noted above, the data rate representing the color video signal is approximately 216 Mbps (megabits per second). When the data in the horizontal and vertical blanking intervals are removed, the number of effective picture elements of the luminance signal per horizontal interval and the number of effective picture elements of each color different signal per horizontal interval become 720 and 360, respectively. Since the number of effective scanning lines for each field in the NTSC system (525/60) is 250, the data bit rate Dv can be expressed as follows: EQU Dv=(720+360+360).times.8.times.250.times.60=172.8Mbps
In the PAL system (625/50), since the number of effective scanning lines for each field is 300 and the number of fields per second is 50, it is apparent that the data bit rate is the same as in the NTSC system.
If redundant data components necessary for error correction and formatting are considered, the total bit rate for the picture data becomes approximately 205.8 Mbpps.
Further, the amount of audio data Da is approximately 12.8 Mbps, while the amount of additional data Do, such as data representing an interblock gap, a preamble, and a postamble used for editing, control and the like, is approximately 6.6 Mbps. Thus the bit rate of all the data to be recorded can be expressed as follows: ##EQU1##
In order to record this amount of information data, the known D1 format digital VTR employs a segment system having a track pattern made up of ten tracks for each field in the NTSC system, or made up of twelve tracks for each field in the PAL system.
In these digital VTRs, a recording tape having a width of 19 mm is used. There are two types of recording tapes which respectively have thicknesses of 13 .mu.m and 16 .mu.m. To house these tapes, there are three types of cassettes, which are respectively known as large, medium and small sizes. The information data is recorded on such tapes in the D1 format with a tape area for each bit of data of approximately 20.4 .mu.m.sup.2. When the recording density is increased, errors tend to take place in the playback output data due to interference between codes or non-linearity in the electromagnetic conversion system of the head and tape. Heretofore, even if error correction encoding is used, the above given value of the recording density has been the limit therefor.
Taking the above described parameters into account, the recording capacities of the cassettes having the various sizes and the two tape thicknesses, when employed in the D1 format digital VTR, are as follows:
______________________________________ Size/Tape Thickness 13 .mu.m 16 .mu.m ______________________________________ small 13 minutes 11 minutes middle 42 minutes 34 minutes large 94 minutes 76 minutes ______________________________________
It will be noted that the recording capacities of these cassettes are relatively short, the longest being just over 11/2 hours. It has accordingly been desired to increase the recording capacity of tape cassettes used for digital video recording by increasing the recording density. One approach to increasing the recording density is to reduce the track width of the recording tracks in which the digital signals are recorded. For example, if the track width were reduced by one-half, e.g. from 10 .mu.m to 5 .mu.m, the recording density, and hence the tape cassette recording capacity, can be doubled. A way of reducing the track width is to use a recording head that has a narrower width. However, use of a narrower head and the resulting recording in narrower tracks, would cause a lack of compatibility between VTRs using different sizes of magnetic heads. That is, tapes recorded on a conventional VTR using a wider head could not readily be reproduced by a VTR using a narrower head, and, conversely, recordings made using a narrower head could not readily be reproduced by conventional, wider head, VTRs.
Further, if it were desired to standardize on a narrower head and track width, it may be that a VTR using the narrow head would perform satisfactorily only if it remained stationary and in a good recording environment and would fail to perform satisfactorily if it were to be transported or used in a location where vibration or other conditions tend to interfere with recording. In such a case, standardization on the narrower widths would not be possible, and a VTR using the wider head, and recording and reproducing wider tracks, would continue to be necessary for non-stationary applications with a resulting problem of compatibility as described above.
Alternatively, standardizing on a wider track and head width of 10 .mu.m would prevent the desired increase in recording density and recording capacity.